The highest court in Massachusetts has rejected the argument of out-of-state same-sex couples that they should be allowed to marry in Massachusetts even though their home states don't recognize such marriages. Problem with their argument is, a 1913 Massachusetts statute forbids such marriages. I haven't reveiwed the decision yet, so I can't say whether this is the right call as a matter of state statutory or constitutional law.
What I can say is that it is the pro-federalism result, buttressing the pre-existing power of the states to take their own path on the issue of gay marriage. Many opponents of gay marriage have argued that we need a federal marriage amendment to prevent gay couples from getting married in jurisdictions that recognize such marriages (for now, just Massachusetts) and then returning to their own home states to demand full recognition ("evasion" marriages). This argument was never a very good one, and today's decision reduces its force even more.
Of course, the "evasion" marriages foreclosed by today's decision are not the only means by which inter-state recognition of gay marriage might spread. The fact that some 6,000 gay couples are now married in Massachusetts guarantees that some of them will move out of the state and ask their new states to recognize their marriages, or at least ask that their marriages be recognized for some purposes (e.g., in a custody or property dispute). These cases will raise different, and much harder, questions than the one disposed of today.
But one part of the argument for a federal amendment has now been weakened. To the extent the decision today releases some of the hydraulic pressure for a federal amendment, what looks superficially like a small defeat for gay-marriage advocates may turn out to be a small victory for them.